Durable, wear-resistant punches and dies

ABSTRACT

Tough, wear-resistant dies derived from cold work steel formed by powder metallurgy processing and containing not greater than about 4% of tungsten. The dies desirably contain not greater than about 4% molybdenum by weight and at least about 5% chromium by weight. Disclosed also is a punch and die combination, the die being derived from cold work steel formed by powder metallurgy processing and containing not greater than about 4% of tungsten. The punch, which may also be derived from cold work steel formed by powder metallurgy processing and containing less than about 4% by weight of tungsten, may have a formula different from that of the die. A punching method is also disclosed.

FIELD OF THE INVENTION

The invention relates to punches and dies used in punching operations, and particularly to dies that are tough and wear resistant.

BACKGROUND OF THE INVENTION

In the fabricating industry, punches and dies are employed to appropriately pierce and shape metal work pieces, the dies having openings for the reception of the tips of the punches during a punching operation. Dies in particular are subjected to substantial and repeated stresses. As the punch tips of punches are forced under high loads through the thicknesses of work pieces and into the die openings, they not only exert compressive forces on the dies in the direction of travel of the punches, but also exert tensile stresses on the dies as the work pieces above the die openings are deformed and press outwardly on the die portions defining the die openings. Dies fail through catastrophic breakage, or because the normally sharp edges of the die openings become worn through repeated usage and considerable regrinding to the point where the dies become unusable.

Accordingly, toughness and wear-resistance are two desired qualities for dies, and this is particularly true for dies, and for punches for that matter, that are adapted for turret punching operations as compared to progressive punching operations. The stresses encountered by dies in turret punching operations may differ from the stresses associated with progressive punching operations, due at least in part to the size and configuration limitations of dies and punches imposed by the geometry of turret punch presses. Also, progressive punching operations are commonly carried out continuously in a manufacturing operation on the same type and thickness of workpiece material (e.g., on a continuous strip of sheet steel drawn from a supply roll), and the punches and dies can be selected to have only the necessary properties of durability and wear resistance for that particular grade or type of workpiece. However, turret punching operations commonly are carried out on a variety of workpiece materials of varying thickness for different projects, and as a result, the dies and punches that are selected desirably have increased durability and wear resistance so as to enable them to be successfully used with the thickest and toughest to machine workpieces likely to be encountered.

Classifications of tool steels include high speed steels and cold work steels, and dies have been manufactured from both of these tool steel types. Tools derived from high speed steels commonly can operate at high temperatures, e.g., up to about 700-1200° F., and possess good “red hardness”. High speed steels generally contain 3 to 5% chromium and greater than 5% and up to 18% or more of tungsten, the tungsten component contributing to high temperature properties. High speed steels are commonly used for making drills, punches, routers, taps, etc. Cold work steels, on the other hand, commonly contain less tungsten, e.g., not more than about 4% and often less than 2%, and are used for making tools such as burnishing and coining tools and shear blades. Cold work steels do not have the red hardness properties that permit high temperature use. Unless otherwise indicated, the percentages of the various elements are given by weight.

A problem with conventional steels involves the formation of carbides due to the inclusion of carbon and various alloying metals such as chromium, vanadium, and tungsten in steel formulations. Carbon reacts with various alloying elements in a steel-making furnace to form metal carbides. The resulting metal carbides are uniformly distributed in the melt, but as the melt solidifies, carbide particles form and tend to clump or aggregate together. When the resulting material is subsequently worked, as in a roller mill, the carbide agglomerates may line up in the direction of work, forming what are commonly known as carbide “stringers”. Carbides generally are very hard and somewhat brittle materials, vanadium carbide and tungsten carbide being among the hardest. When tool steel blanks are machined to make tools such as punches and dies, the carbide stringers not only make the steel alloy blanks difficult to work with, but also tend to provide fracture lines along which the resulting tool materials will fracture during subsequent use. Microscopically, high carbon steels commonly exhibit a grain structure in which the carbide stringers show up prominently.

Powder metallurgy makes use of a different process of forming tool steel alloys. The melt, containing molten iron, carbon, and various alloying elements such as vanadium, chromium and molybdenum, is formed in the usual way. Thereafter, however, the molten material is atomized—that is, it is formed in a known manner into small droplets. Each of the resulting droplets or particles, then, has the same composition as the melt from which it came, that is, each particle has the same atomic make up; it is its own “ingot.” Particles are then placed in a canister and are subjected to intense pressures at temperatures below the melting points of the metals. The particles fuse together without melting to form ingots. Since the melt is not permitted to solidify by itself (which otherwise could give rise to carbide stringers), the resulting powder metallurgy ingot is very uniform in composition, and the carbide portions are contained in the ingot in a very evenly distributed fashion without evidence of stringers.

As a result, powder metallurgy techniques enable alloys of various types to be manufactured that could not be manufactured through routine steel making processes. For example, in routine steel making, it is difficult to obtain a carbide volume concentration greater than about 20%, whereas carbide concentrations up to 30% are not uncommon in powder metallurgy materials.

Punch tools and dies have in the past been formed from high speed steel blanks resulting from powder metallurgy processing, but these tools, and particularly the dies, have not exhibited a desired combination of toughness and wear resistance. Toughness involves the ability of a die to absorb repeated impacts without breaking. Wear resistance, as the term implies, involves how much a die wears upon repeated use. It is commonly understood that harder materials have greater wear resistance. However, harder materials also tend to exhibit greater brittleness, which may lead to reduced toughness and a greater propensity of dies to shatter catastrophically.

SUMMARY OF THE INVENTION

The present invention provides dies that are derived from powder metallurgy processes and that exhibit a combination of excellent wear resistance and toughness.

In one embodiment, this invention provides a durable, wear-resistant die for use in turret punching operations, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing not greater than about 4% tungsten by weight, and preferably not greater than about 2% tungsten by weight.

In another embodiment, the invention provides a durable, wear-resistant die for use in punching operations, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing from about 0.2 to about 4% (and preferably from about 0.5% to about 2%) of tungsten by weight and, preferably, containing from about 5% to about 10% (and most preferably from about 7% to about 9%) of chromium by weight.

In yet another embodiment, the invention provides a durable, wear-resistant punch and die for use in punching operations, the die having a body derived from cold work tool steel formed by powder metallurgy processing and having a first formula containing not greater than about 4% of tungsten by weight, and said punch having a body derived from cold work tool steel formed by powder metallurgy processing and having a second formula containing not greater than about 4% of tungsten by weight, the second formula being different from the first formula.

In another embodiment, the invention relates to a method of forming a metal workpiece on a turret punch press, comprising providing a die in said turret punch press, the die having a body derived from cold work tool steel formed by powder metallurgy processing and comprising not greater than about 4% of tungsten by weight, and performing said punching operation using said die. Preferably, the method includes the step of providing a punch for use with said die, the punch having a body derived from cold work tool steel formed by powder metallurgy processing and comprising not greater than about 4% of tungsten by weight. The elemental powder metallurgy composition of the punch may be different from that of the die.

In a preferred embodiment, the invention provides a durable, wear-resistant die having a body derived from cold work tool steel formed by powder metallurgy processing and containing tungsten in an amount not greater than about 2% by weight, at least about 7% (and preferably at least about 7.3%) chromium by weight, and not over about 2% of molybdenum by weight.

Desirably, the cold work, powder metallurgy-derived steels from which are manufactured dies of the invention contain, independently or in combination, from about 0.9% to about 3% (preferably about 1.5 to about 2%) by weight of carbon, from about 5% to about 10% (preferably from about 7% to about 9%) by weight of chromium, from about 1% to about 4% (preferably about 1% to about 2%) by weight of molybdenum, and about 2% to about 6% (preferably about 1% to about 2%) by weight of vanadium.

Although the invention is particularly desirable in connection with dies, and especially dies used in turret punching environments, it may in some instances be desirable to manufacture not only the die but also the punch (particularly the punch tip) from powder metallurgy materials. Punches, for example, commonly fail by chipping away of a sharpened edge, and good toughness and wear resistance, particularly the latter, are important for punch tips as well.

As used herein, “hardness” commonly is measured on the Rockwell C scale, with Rockwell C values in the range of about 58 to about 62 being desired.

“Toughness”, as used herein, refers to how well a die can resist breakage when subjected to substantial loads; and one measure of toughness appropriate for dies is compressive strength. Dies used in punching operations must resist repeated high compressive loads without breaking. Experience shows that when dies break during use, they break catastrophically, that is, they tend to shatter, and this can lead to damage not only to work pieces but more importantly to the punch presses themselves and to punch press operators.

Wear resistance is measured by actually measuring the distance that a surface wears when subjected to wear-producing forces, this surface commonly being the cutting edge of a die opening.

One die of the invention was manufactured by machining the die from a cold work tool steel blank formed by powder metallurgy from a melt containing 1.1% carbon and 7.75% Cr, the melt containing 1.10% by weight of tungsten and the blank being designated “A” in the following tables. Similar dies were made from other cold work tool steel blanks formed by powder metallurgy and referred to herein as “B”, “C” and “D”, each of these blanks containing less than 4% by weight of tungsten. For comparison, another die was manufactured from a high speed tool steel blank formed by powder metallurgy and containing 5.5% by weight of tungsten, this material being referred to as “1” below. Yet other dies were manufactured from cold work tool steel blanks that were not made using powder metallurgy processing, these dies being referred to below as “2”, “3”, “4” and “5”. The chemical composition in percent by weight of each of these blanks is given in Table I, it being understood that there may be included trace amounts of other materials as well. TABLE I Tool Steel C Cr Mo V W S Si Mn A 1.10 7.75 1.60 2.35 1.10 1.2 0.25 B 0.80 7.50 1.30 2.75 C 1.42 7.57 1.28 3.71 1.75 1.17 0.29 D 1.79 7.61 1.30 5.73 1.74 0.12 1.24 0.41 1 1.42 4.00 5.25 4.00 5.50 0.30 0.30 2 1.00 5.25 1.10 0.25 0.35 0.85 3 1.55 11.5 0.90 0.80 0.45 0.35 4 0.85 4.15 5.00 1.95 6.40 0.30 0.30 5 0.80 7.5 1.50 2.20 0.20 0.95 0.35

The “A” blank material was machined into a 3″×0.75″ die, and was used to punch holes in 0.060″ mild steel sheets. After 25,000 hits, the die was examined and found to need no sharpening, whereas prior art dies normally need to be sharpened at least once after such use. The same material was machined into a 3.5″ special shape die, and was used to punch holes in 0.187″ stainless steel sheets. Dies commonly catastrophically fail within 400 hits, whereas the die fabricated from the “A” blank material did not break or need sharpening after 400 hits.

In the materials below, toughness was measured on 5 inch diameter turret dies manufactured by identical processing but derived from different tool steels, some of which resulted from powder metallurgy processing. Testing was accomplished using a Tinius Olsen L tensile test machine using a compression load applied at a rate of 0.1 inches per minute at loads up to 200,000 pounds. The load was applied until failure occurred, failure being signaled by a cracking sound and production of a crack in the die specimen. Results are reported in Table 2. TABLE 2 Tool Steel Pounds to Failure A 200,000 B 200,000 1 198,000 2 168,000 3 103,000 4 98,000 5 163,000

Wear testing of dies manufactured by identical processes but derived from different tool steels was accomplished using a Nisshinbo Model 1250 turret punch press. Identical punches were employed, and repeated punching through sheet metal work pieces produced measurable wear on the cutting edges of the dies, that is, the sharp edges of the die openings. The dies were subjected to an equal number of hits, and the dies were checked for edge wear. Results are reported in Table 3. TABLE 3 Tool Steel Average Wear, Inches A 0.0015 B 0.0020 3 0.0035

While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims. 

1. A durable, wear-resistant die for use in turret punching operations, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing not greater than about 4% of tungsten by weight.
 2. The die of claim 1 wherein said tool steel contains at least about 5% chromium by weight.
 3. The die of claim 1 wherein said tool steel contains not more than about 4% molybdenum by weight.
 4. The die of claim 1 wherein said tool steel contains from about 0.9% to about 3% carbon by weight.
 5. A durable, wear-resistant die for use in punching operations, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing from about 0.5% to about 2% of tungsten by weight.
 6. A durable, wear-resistant die for use in punching operations, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing tungsten in an amount by weight not greater than about 2%, not greater than about 2% molybdenum by weight, and at least about 7% chromium by weight.
 7. A punch having a punch tip and a die having an opening receiving the punch tip in a punching operation, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing not greater than about 4% of tungsten by weight.
 8. The punch and die of claim 7 wherein said cold work tool steel contains at least about 5% chromium by weight.
 9. The punch and die of claim 7 wherein said cold work tool steel contains not more than about 4% molybdenum by weight.
 10. The punch and die of claim 7 wherein said tool steel contains from about 0.9% to about 3% carbon by weight.
 11. The punch and die of claim 7 wherein both said punch and said die are derived from said cold worked steel.
 12. The punch and die of claim 7 wherein said punch is derived from cold work tool steel formed by powder metallurgy processing and that has a formulation different from the tool steel from which the die is derived.
 13. The punch and die of claim 12 wherein said punch is derived from a tool steel containing tungsten in an amount not exceeding about 4%.
 14. A punch having a punch tip and a die having an opening receiving the punch tip in a punching operation, the die having a body derived from cold work tool steel formed by powder metallurgy processing and containing not greater than about 2% of tungsten, not greater than about 2% molybdenum by weight and at least about 7% chromium by weight.
 15. The punch and die of claim 11 wherein said tool steel contains from about 1% to about 2% carbon by weight.
 16. A method of forming a metal workpiece on a turret punch press, comprising providing a die in said turret punch press, the die having a body derived from cold work tool steel formed by powder metallurgy processing and comprising not greater than about 4% of tungsten by weight, and performing said punching operation using said die.
 17. The method of claim 16 including the step of providing a punch for use with said die, the punch having a body derived from cold work tool steel formed by powder metallurgy processing and comprising not greater than about 4% of tungsten by weight.
 18. The method of claim 17 wherein the elemental powder metallurgy composition of the punch is different from that of the die. 